Neil,
If you get weather in the Berg where you can use a 4 season sleeping bag, please invite me along as I’d love to test out my 4 season bag I took to Nepal.
Bottom line - it rarely (if ever) gets that cold in the Berg. I use a TNF Blue Kazoo 650 down bag rated at -10 degrees and it’s perfect. Ok, I’m also a warm sleeper, but still, chances of it ever getting cold enough to use a 4 season bag for you are slim. bag can compress to about 15cm high and about a 15cm diameter as well with the fitted compression bag.
Liners…mmmhh, agree with above sentiments. Use mine mostly to keep the sleeping bag clean. Don’t actually use it anymore, just wash/wi[e off all dirt before I climb into the bag. Also, I use tights (pants and top) so dirt isn’t much of an issue. This probably adds more warmth than a bag liner 
. Seriously, they do add a couple of degrees, maybe 3 -4 deg C but I seriously doubt any more than this. So for the additional space they take, and the additional weight I’d rather leave them home. I usually take a bivvy bag along - mostly for wet conditions, but this little device is much more effective in keeping me warm than a bag liner would ever be. Same weight, a bit bigger but can be used for different purposes. And then I can save weight on a tent as well